Prior art fluid purification and separation processes typically include passing a fluid stream which includes an impurity through a first adsorption zone. The impurity is adsorbed on the adsorbent material of the first adsorption zone and a purified fluid or a product fluid is discharged from the first adsorption zone. The fluid stream is passed through the first adsorption zone until the adsorbent material nears impurity saturation, and the impurity being adsorbed nears breakthrough into the product fluid.
During the period that the first adsorption zone is on stream for the adsorption of impurity, a second adsorption zone which has previously been on stream is being desorbed to remove the impurity from the adsorbent material of the second adsorption zone. Thus, when the adsorbent material of the first adsorption zone nears impurity saturation, the flow of the fluid stream through the first adsorption zone is terminated, and the fluid stream is passed through the second adsorption zone. Thereafter, the second adsorption zone is on stream for the adsorption of impurity while the first adsorption zone is desorbed to prepare it for subsequent use on stream.
Factors which complicate the ostensibly simple cycling of the adsorption zones between adsorption and desorption cycles are that the discharge of product fluid should be continuous and the number of adsorption zones should be minimized. The selection of a cycle is further hampered by the face that the desorption cycle includes several different phases. For example, adsorption is tyically carried out at a first relatively high pressure, and the first phase of the desorption cycle may be the depressurization of the adsorption zone. Depressurization may be used to recover or otherwise dispose of some of the fluid trapped in the adsorption zone to the extent that such fluid is as pure as the feed fluid.
In the second phase of the desorption cycle, the impurity previously adsorbed from the adsorbent material is removed. This may be accomplished by further reducing the pressure in the adsorption zone. In order to maximize impurity removal, this phase of the desorption cycle should be as long as possible and preferably at least as long as the period of adsorption.
Finally, in the third phase of the desorption cycle, the adsorption zone must be repressurized back to approximately the first relatively high pressure. The repressurization makes the adsorption zone ready for subsequent adsorption.
These and other considerations have brought about the use of adsorption systems which include at least three adsorption zones. Typical of this are the processes disclosed in U.S. Pat. No. 3,176,444 issued to Kiyonaga. However, with the patented processes either the impurity removal phase is relatively short in which event impurity removal is not maximized or up to five adsorption zones are required. In addition, except for systems having four or more adsorption zones, the vacuum equipment of the Kiyonaga system is not used continuously, and this results in inefficient utilization of the vacuum equipment.